The
years 1860-1870 announced and inaugurated a period of renewal for the metric
system. All scientists recommended its use, like the Englishman Maxwell or
the German Grauss. After the 1867 International Exhibition, a Committe
stated that "the metric system can totally be adopted…in science, art,
industry and trade". In 1870, it was the only legal system in thirteen
countries, it was the basis to standards of eight other countries, and its
use was legally allowed in the majority of the others, from which the United
States.
At
a metrological level, a decimal system based on the Archives standards was
recommended. In August 1870, at the French government's request, a "Metre
International Commission" met in Paris. Because of the war, it only
appointed a preparatory committee. Works were resumed in 1872 with the
participation of twenty-eight countries. On 20 May 1875, the representatives
of seventeen countries, out of the twenty countries that were present,
signed the "Metre convention".
This
Convention created a set of organisms entrusted with ensuring the uniformity
of physical measures worldwide. These organisms are the "International
Bureau of Weights and Measures" (IBWM), a permanent metrology laboratory
situated in Sevres, near Paris; the "International Committee for Weights and
Measures" (ICWM) which controls directly the IBWM and prepares decisions and
recommendations; lastly, the "General Conference of Weights and Measures" (GCWM),
a higher jurisdiction that calls together the representatives of the
Convention signatory states every fourth year.
The
first General Conference held in 1889 sanctionned the realization of
international prototypes that were "deduced from the values of the Legal
Metre and Kilogramme, as they stand at present". These prototypes are made
of an alloy of platinum and ten per cent of iridium.
They
were made with drastic precautions which would have been unthinkable a
century earlier. The Legal standards platinum had many impurities. It was
made up of platinum powder that was agglomerated by forging. The new alloy
was melted; it was analyzed carefully to check that there were no oxidizable
or magnetic impurities like iron or ruthenium. The new standards were
produced between 1871 and 1888, firstly under the responsibility of the
French Section of the Metre International Commission, and especially of the
chemist H. Sainte-Claire, of Deville and of the physicist H.Fizeau who
themselves carried out many checks, then under the responsibility of the
ICWM and the IBWM.
The
1889 international Kilogramme is a cylindre. It is still nowadays the mass
prototype. The 1889 international Metre is a x-section ruler. It is 102 cm
long, and has a line at one centimetre from each end. The distance between
these two lines define the metre.
The
practical metre length is still the one given by this prototype, even for
measures that demand a high degree of accuracy.
Mètre et kilogramme étalons of 1889 (IBWM)
However,
the accuracy allowed by this material standard has became inadequate for
some present needs, and it was thought necessary to modify the metre's
definition, by adopting an "optical standard". A wavelength was chosen as a
basis in 1960. The definition changed again in 1983 when it was linked to
the speed of light.
Nowadays,
another two international organisations work towards the implementation - at
pratical and legal levels - of the GCWM. Theses organisations are the
"International Organisation of Legal Metrology" (IOLM), in charge of the
international harmonization of legislation relating to units of measure and
the "International Standards Organization" (ISO) in charge of standardizing
the use and the writing of the SI units symbols - SI standing for
International System of Units.